Wireless data acquisition network and operating methods

ABSTRACT

A wireless network is provided, that may comprise wireless sensor units organized in chains of wireless sensor units. Each wireless sensor unit may comprise plural sensors and at least a wireless transceiver connected to communicate by wires or wirelessly with the plural sensors. Each chain of wireless sensor units may include a terminal wireless sensor unit and intermediate wireless sensor units, each intermediate wireless sensor unit being configured to relay data along the chain of intermediate wireless sensor units towards the terminal wireless sensor unit. The terminal wireless sensor unit in each chain of wireless sensor units is adapted to communicate wirelessly with at least one backhaul unit of plural backhaul units; and the backhaul units are adapted to communicate with a central computer.

FIELD

Wireless data networks and their operation.

BACKGROUND

Seismic surveys are extensively used in the oil and gas industry tounderstand the subsurface and to provide structural images of thegeological formation within the earth using reflected sound waves. Theresults of the survey are used to identify reservoir size, shape anddepth as well as porosity and the existence of fluids. Geophysicists andgeologists use this information to pinpoint the most likely locationsfor successfully drilling for oil and natural gas.

The seismic survey is conducted by placing a large number of geophonesin the area of interest. They are set up in lines or in grids. Usingshakers or small explosives, the ground is shaken and the geophonesacquire the reflected sound data from the different sub-layers in theground. A huge amount of data is collected in a given seismic surveywhich can cover 40 sq km and take days to gather.

The amount of data which is retrieved during a seismic survey is quitelarge. In an exemplary case a geophone measures three axes at a samplingrate of 4 bytes per millisecond (each byte is 8 bits giving a resolutionof 24 bits which is the accuracy required by the seismic survey). Inthis case, the data rate per geophone is:4 bytes/msec×8 bits/byte×3=96 kbps (Kilobits per second)

If the survey is using 1000 geophones, the data rate is then 96 Mbps(Mega bits per second). Because wireless systems have overhead and errorcorrection to operate reliably, even the highest data rate broadbandwireless systems can't accommodate this data rate in traditionalconfigurations such as point to multipoint or pure mesh systems.

Several patents are known that use wireless links in a seismic network,including U.S. Pat. Nos. 6,424,931; 6,041,283; 6,219,620; and 7,224,642.However, there is room for improvement in the manner in which data iscollected and delivered for processing.

SUMMARY

Methods and apparatus for collecting data from a wireless sensor networkare provided. The methods and apparatus apply for example to seismicnetworks, but may be applied to other wireless sensor networks.

In one embodiment, a wireless network is provided, that may comprisewireless sensor units organized in chains of wireless sensor units. Eachwireless sensor unit may comprise plural sensors and at least a wirelesstransceiver connected to communicate by wires or wirelessly with theplural sensors; each chain of wireless sensor units including a terminalwireless sensor unit and intermediate wireless sensor units, eachintermediate wireless sensor unit being configured to relay data alongthe chain of intermediate wireless sensor units towards the terminalwireless sensor unit; the terminal wireless sensor unit in each chain ofwireless sensor units being adapted to communicate wirelessly with atleast one backhaul unit of plural backhaul units; and the backhaul unitsbeing adapted to communicate with a central computer.

In another embodiment, a method of collecting data from wireless sensorunits arranged in a network is provided in which the wireless sensorunits are organized in chains of wireless sensor units. Each chain ofwireless sensor units may include a terminal wireless sensor unit andintermediate wireless sensor units. The method may comprising the stepsof initiating distribution of control signals to the wireless sensorunits; acquiring data with the wireless sensor units by sensing one ormore physical parameters; each of the wireless sensor units transmittingthe acquired data in response to the control signals along at least oneof the plural chains of wireless sensor units towards a correspondingone of the terminal wireless sensor units; each of the terminal wirelesssensor units forwarding the acquired data from the wireless sensor unitsin the corresponding chain to at least one of plural backhaul units; andeach of the backhaul units collecting and forwarding the acquired datafrom the terminal wireless sensor units towards a central computer.

Methods of prioritizing sending of data in wireless sensor networks arealso provided.

These and other aspects of the network and method are set out in theclaims, which are incorporated here by reference.

BRIEF DESCRIPTION OF THE FIGURES

Embodiments will now be described with reference to the figures, inwhich like reference characters denote like elements, by way of example,and in which:

FIG. 1 shows an exemplary wireless sensor unit which includes a wirelesscapable data collection box connected to several sensors, in this case,geophones.

FIG. 2 shows an exemplary distribution of wireless sensor units in awireless mesh network.

FIG. 3 shows a 2D wireless network with wireless links to feed data backto a control van.

FIG. 4 shows a 3D wireless network with wireless links to feed data backto a control van.

FIG. 5 shows a wireless network with balloon based coverage.

FIG. 6 shows a wireless network operation.

FIG. 7 shows an example of arbitrary grouping of wireless sensor unitsthat allow the operator to selectively pull data from an arbitrary groupof boxes.

FIG. 8 shows an algorithm for prioritizing sending of data from wirelesssensor units over a wireless network, in this case using a randomback-off algorithm.

FIG. 9 shows an algorithm for prioritizing sending of data from wirelesssensor units over a wireless network, in this case using a greedydownload algorithm.

FIG. 10 shows a scenario in which a greedy download algorithm may beused.

FIG. 11 shows an algorithm for prioritizing sending of data fromwireless sensor units over a wireless network, in this case sending highpriority data first, then following with low priority data.

FIG. 12 shows a data prioritization method: Send critical ‘X’ datafirst, and then follow later (or offline) with low priority ‘Y’ and ‘Z’data.

FIG. 13 shows a data prioritization method using interlaced datasamples: Send “320” data first, then follow later with “330” data.

FIG. 14 shows a data prioritization method using resolution reduction:Send “420” data first, then increase the resolution later by sending“430” data.

FIG. 15 shows a combination of prioritization methods: Send High bits(520) of interleaved words (540), then follow later filling in withhigher resolution and more data.

FIG. 16 shows a method of awakening wireless sensor units from sleep.

FIG. 17 shows a method of a wireless sensor unit transitioning betweendormant and active.

DETAILED DESCRIPTION

FIG. 1 illustrates a wireless sensor unit 20. Many different wirelesssensor configurations may be used. In the example shown, wireless sensorunit 20 comprises several sensors 22, in this case six, connected bywires to a data collection box 24. The data collection box 24 is wiredto a wireless transceiver 26. In some embodiments, instead of sixsensors, there may be one or more sensors. In some embodiments, thewired connections shown in FIG. 1 may be wireless. In some embodiments,any combination of the sensors 22, data collection box 24 and wirelesstransceiver 26 may be incorporated in a single housing. In someembodiments, the sensors 22 may be geophones.

In FIG. 1, the data collection box 24 is connected to six digitalgeophones 22 capable of sampling data from three directions—X, Y, and Z.There are 3 geophones 22 connected in series and attached to each sideof the box 24 via a cable. Each geophone 22 may for example sample thesignal at 1 millisecond (ms.) interval, and each sample consists of 4bytes of data per millisecond. The wireless transceiver 26 may beconnected to the data collection box 24 via RS-232 and SCSI ports. Theseconnections may be generalized to other types of ports including (butnot limited to) USB, FireWire, parallel, synchronous serial, etc.

Since there are 3 directions of motion sampled simultaneously, the dataacquisition rate per geophone is4 bytes/msec×8 bits/byte×3=96 kbps (Kilobits per second)

With 6 geophones 22 attached per box 24, the data rate received by a boxis therefore 576 kbps.

In a seismic network, the acquisition may occur over a period rangingfrom a minimum of 5 seconds to a maximum of 20 seconds, each acquisitionfollowed by an 8 second interval. Therefore the data rate to betransmitted back to the central control unit will have a minimum datatransmission rate of

$\frac{576\mspace{11mu}{kbps} \times 5\mspace{11mu}\sec}{5 + {8\mspace{11mu}\sec}} = {222\mspace{11mu}{kbps}}$

And a maximum of

$\frac{576\mspace{11mu}{kbps} \times 20\mspace{11mu}\sec}{20 + {8\mspace{11mu}\sec}} = {412\mspace{11mu}{kbps}}$

Therefore the maximum data rate for real time data transmission is 412kbps per box 24. Given that there are sometimes hundreds of boxes 24active at the same time, the amount of data flowing simultaneouslythrough the network at the time of acquisition is quite substantial andrequires careful management of the data flow throughout the network.Embodiments disclosed here are intended to provide real-timetransmission of the data if required. Real-time in this case beingdefined as data transmission occurs substantially during the seismicdata acquisition period. The network may offer the user the ability totailor the performance to their specific requirements and their specificapplication. In some cases, they may not need to meet the real timerequirement and the system enables the user to utilize it in a non-realtime way.

In an embodiment, a hybrid wireless mesh/cellular network configurationmay be used with standards-based equipment. Equipment based on thestandards can be configured to operate at different frequencies, thusallowing for extremely high data rates while avoiding problems ofself-interference or limited bandwidth. However, any type of radio maybe used which has the capability to transmit the amount of data requiredand is capable of being connected in mesh or cellular networks. As anexample of a custom radio with such capabilities, Wi-LAN's VIP 110-24 iscapable of forming a mesh network as required by this invention eventhough it is not based on a standard product.

In an embodiment, a Hybrid Mesh network system uses wireless units basedon IEEE 802.11g standard capable of transmitting 36 Mbps (we will use aneffective bandwidth of 27 Mbps to account for overhead) of data todistances determined by the following Friis equation:P _(r) =P _(t) G _(t) G _(r)(λ/4πR)², where:

P_(r) is the received power,

P_(t) is the transmitted power,

G_(t) is the transmitter gain,

G_(r) is the receiver gain,

R is the distance between routers, and

λ is the signal wavelength.

With 20 dBi transmit power, 36 Mbps links up to 500 m away can beattained within the advertised required fade margin of a typicaloff-the-shelf router, or, 24 Mbps links may be attained up to 1,000 mwith a fade margin within the advertised required fade margin of atypical off-the-shelf router. By using a mesh configuration, each box isable to receive data from another box in its immediate vicinity andre-transmit it to a box further down the line until all the data fromall the boxes reaches its final destination. Boxes that are downstreamfrom others have to transmit their own data as well as the data receivedfrom the boxes located upstream from them. At the network level, thismeans the total transmit time is equal to approximately half the timeavailable. As a result the total number of boxes that can be connectedin a single line or in a mesh configuration could be limited to about 32boxes. Thus for every 32 boxes we will need a backhaul unit to get thedata back to the central control unit. Furthermore, because of certainsystem inefficiencies, we may have to limit this to a maximum of 25boxes per each backhaul unit. A backhaul unit comprises a transmitterand a receiver with highly directional antennas. These are clearlyexemplary numbers.

A mesh network architecture may be used either to connect geophonesdirectly to each other, or to relay boxes, which are in turn connectedto a data collection van some distance away. In some embodiments, higherlevel mesh networks are overlaid upon these local mesh networks in abackhaul network based on a more cellular approach.

A hybrid mesh network configuration is very well suited for seismicsurveys. In one embodiment, the network self-configures into linearmeshes by means of each unit discovering and connecting to its nearestneighbors. All of the mesh units may use omni-directional antennas(antennas which cover 360 degrees of azimuth). If deployed, the backhaulunits may use directional antennas with relatively wide beamwidths, say,90 degrees or 120 degrees, to increase antenna gain and reduceinterference. This allows them to be pointed in generally the rightdirection but without requiring an enormous amount of setup.

In FIG. 3, wireless sensor units 20 in plural chains of wireless sensorunits 20 in a wireless mesh network 29 communicate with each other andone of the wireless sensor units 20 communicates with a correspondingbackhaul unit 28. While the backhaul unit 28 is shown more or lesscentrally in relation to its corresponding chain of wireless sensorunits 10, the backhaul unit 28 may be located at the end of itscorresponding chain, as in FIG. 6 or in any other suitable location. Thebackhaul units 28 in turn relay data from the wireless sensor units to acentral control unit 36. In this case, IEEE 802.11g may be used for themesh network 29 and IEEE 802.11a may be used for backhaul.

One implementation involves connecting a number of geophones to eachother using short-range radios, and a relay box may incorporate acompatible short-range radio to collect the data from locally placedgeophones and relay the data via a second mesh based on a longer-rangeradio (e.g. IEEE 802.11g). The maximum number of relay boxes is thendetermined by the capacity of the longer-range radio. When that numberis reached, a backhaul unit would then transmit all the data collectedin either a cellular network or a point-to-point link (e.g. IEEE 802.11aor 802.16) back to a computer at the control truck for collection,storage and interpretation. The backhaul units would also have tworadios: one radio to communicate with the last relay box in the mesh andthe second radio to relay data to the control truck.

In FIG. 4, a 3D network is shown in which wireless sensor units 20communicate through respective backhaul units 28 (see FIG. 3), and thebackhaul units communicate with a control truck 36. In this case,short-range radio such as IEEE 802.15.4 may be used to provide awireless connection between the geophones and the box in each wirelesssensor unit 20, longer-range radio such as IEEE 802.11g may be used forthe mesh network (communication between wireless sensor units 20), andan alternative system on a separate channel, such as IEEE 802.11a may beused for backhaul to the control truck 36.

Another method is shown in FIG. 5, in which a tethered (or un-tethered)balloon carrying carries a wireless mesh node. The node 30, functioningas a backhaul unit, attached to the balloon 32 would be within range ofthe sensor units 20 in a large geographical area 34 and therefore wouldprovide excellent coverage of rough terrain. Each node 30 communicateswith the control truck 36 (see FIG. 6).

The wireless sensor unit 20 may use off the shelf components modifiedaccording to the disclosed methods. The mesh network provides a virtualconnection between the control van and each data collection box in thefield.

The hybrid mesh network may be applied both to existing analog geophonesand to future digital geophones. Future implementations may require theuse of short-range radio equipment to create the small local networkusing a number of geophones. One such short-range radio is the IEEE802.15.4 (ZigBee) system. Although the 802.15.4 system has too low athroughput and range to handle the full survey, it has enough capabilityto connect a few geophones 22 (which are less than 100m apart and whichrequire an aggregate data throughput of less than 256 kbps, a relativelylow data rate). Thus, in one embodiment, a limited number of geophones22 are connected to each other, transmitting all their data to a box 24with a higher-capacity (e.g., 802.11g) radio 26. The box 24 incorporatesmemory to buffer the data. This memory could be a hard drive, flashmemory, or some other storage device, depending on the requirements ofthe system. The boxes 24 are then connected to their own mesh network asdescribed above, and the data is backhauled using either other channelsof the local system (e.g., 802.11g) or some other (IEEE 802.11a, IEEE802.16, cellular) backhaul.

In the case where the system is being retrofitted to analog geophones,the radios 26 or the data control box 24 may incorporate anAnalog-to-Digital converter to enable the digitization of the data fromthe geophones.

In FIG. 6, a method of operation of the network is disclosed. At t1 theodd numbered radios 38 transmit while the even numbered ones 40 receive.In this example, the backhaul radio 28 is receiving therefore thecontrol truck 36 radio is idle. Then at t2, the even numbered radios 40are transmitting and the odd numbered ones 38 are receiving. In thiscase the last radio 42 in the line is idle, while the backhaul radio istransmitting the data it has collected to the control truck. Then at t3the same situation as t1 occurs and so on until all the data has beentransmitted back to the control truck.

As shown in FIG. 6, when the network self-configures after installation,each radio may be allocated a number representing its location in eachline. For example, radio number 25 in line 1 would be allocated “1-25”.This numbering may start from the radio closest to the backhaul unit,which would be number xx-1. As soon as the seismic acquisition begins,all odd numbered radios begin transmitting and all even radios receive.Radio number 1 transmits to the backhaul unit. Once they havetransmitted all their data, the odd numbered radios receive while theeven ones transmit. The transmission is always from number N to numberN−1. Thus, the data is passed on from one radio to the next. The radiosdownstream effectively relay all the data for their system as well asall other systems upstream from them. Each unit has sufficient datastorage to buffer the data it receives prior to the next transmission.This process continues until all the acquired data has been collected atthe control truck. Control truck 36 is shown for each chain of wirelesssensor units 20, but generally, there will be a single control truck 36,with a computer system, and one or more radios as required by theconnection to the backhaul units. The connection to the backhaul unitsmay be wired or wireless.

Another technique is to group seismic boxes 20 based on their locationin the mesh. This technique is shown in FIG. 7. In order to reduce meshrelay loads, groups 44 are configured so that data sent from a remotebox to the control van uses a “quiet” path through the mesh and throughbackhaul units 28 (FIG. 3). This way, several boxes may download theirdata without causing excessive collisions in the network.

One issue with wireless replacement of data collection cables is thelarge amount of bandwidth required for every seismic data record.Immediately following each seismic event (usually triggered by anexplosive device or other physical means to send a seismic shock waveinto the area being measured), a very large amount of data is ready fortransmission back to the data collection equipment. Because all of thesensors are triggered by the same event, the wireless network suddenlygoes from a quiescent state to near or beyond capacity. FIG. 2 shows amesh 130 of sensor units 20 triggered by an event 120 so that most ofthe units have data and are active 110, while some units 140 may nothave received data or may have already transmitted their data.

In order to prevent the overloading of the network, a random back-offmay be used in some embodiments to stagger the start of transmission foreach set of sensors in the array. The unique condition where a startingpoint is well defined allows a predictable uniform distribution of dataupload start times throughout the array of sensors. Each mesh devicedelays in step 54 transmission of its own data for an amount of timegenerated randomly in step 52, based, for example, on the device'sserial number or a timer. Timing may be initiated at the detection instep 50 of the start of a new seismic event. Although the device may berelaying data from other parts of the mesh, during the timer period, thedevice does not inject its own data into the mesh. Once the time-outperiod has been reached, the device begins in step 56 to transmit itsown data (in addition to relaying any other data passing through themesh) to the control van.

The Usage is generally extensible to any triggered event that will causea significant increase in bandwidth usage due to the plurality ofdevices all attempting to communicate simultaneously (triggered by theevent). The algorithm for this technique is shown in FIG. 8.

There are cases where the control may initiate a full download of allmesh data. Using a broadcast message, each box is instructed in step 60to transmit all of its data back to the control van. The execution ofthis command would result in a flood of network traffic and a very highcollision rate. In order to reduce the collision rate, each box employsa “greedy” technique to ensure that its data is transmitted before ithas to relay data from other boxes.

To employ this technique, the box examines broadcast messages beforerelaying them to nearby nodes. If in step 62 the broadcast message is adownload message and the box has data to download, the box initiates thedownload procedure 56 but does not relay in step 64 the broadcastmessage until in step 66 its download is complete.

PERFORMANCE EXAMPLES Example 1

If each mesh node 70 is within range of two other nodes, and the controlvan 36 is within range of four mesh nodes, this technique results in alinear download where only one node in each chain is downloading at atime. Nodes further down the chain are idle, and nodes between thedownloading box and the control van are relaying one box's data. At thecontrol van, the four mesh nodes within range would be simultaneouslytransmitting data, which is a very light load.

Example 2

If each mesh node 70 is within range of three other nodes and thecontrol van 36 is within range of four mesh nodes, the first four nodesin the chain download their data before passing the broadcast message tothe links following in the chain. If each node reaches two other nodesfurther down the chain, this results in eight nodes simultaneouslytransmitting at the second level, and the amount of data simultaneouslytransmitted doubles with each additional layer. In this scenario, ifthere are two hundred (200) boxes in the field, the first download cyclehas four devices transmitting simultaneously, the second cycle haseight, the third has sixteen, and so on. After five cycles ofdownloading and broadcast message forwarding, the field of devices hasdownloaded all data. On the fifth cycle, 64 boxes are simultaneouslytransmitting back to the control van. On the sixth cycle, 76 boxes aresimultaneously transmitting back to the control van. This technique isillustrated in FIG. 10.

Although Example 2 still results in a large link load as the broadcastmessage spreads through the network, it is still far better than if thebroadcast message reached all nodes at roughly the same time. Further,the most typical network configuration is that of Example 1, where nodesare chained in a linear fashion and rarely can a given node see morethan the node linking it to the control van and one other node furtherout in the field. Finally, if there are many other nodes in thevicinity, other transmission reduction techniques outlined in thisdisclosure can be used to reduce the network load.

This technique may be extended to groups within the network. A group ofboxes may be defined as a subset of the entire network. This group maybe a cluster physically located near each other, it may be a set ofstrategically placed nodes throughout the network, or it may be someother arbitrary grouping. The broadcast message may still be required toinitiate download, but nodes not in the group would simply relay themessage. Allowing grouping provides more flexibility for data retrievaland also presents another technique for reducing network load. If, inthe above Example 2, there were four groups defined for the 200 nodes,the size of each group is 50 nodes. The maximum download load would thenbe 22 simultaneously transmitting nodes. Once the operator has finishedreceiving data for a group, a broadcast message may be sent, initiatingdownload for a new group.

Transmission Load Reduction Using Interlacing

The interlace techniques defined here are methods of reduction of theinitial amount of data to be transmitted, while maintaining a quality ofinformation that will be useful to an operator to determine an estimatedquality of the incoming data as well as knowledge that the sensors andsystem are operating as expected. This may be accomplished using one ormore of the specific techniques listed below. Generally, the method isto prioritize specific data to be sent immediately, and remaining datais sent on a low priority basis. Selecting the higher priority datausing one of the methods listed below will allow the operator to receivereal time information on the current shot without burdening the networkwith lower priority data. Once the network settles down from the initialburst of data, the lower priority data can be sent for a more detailedor higher resolution picture of the seismic measurements.

FIG. 11 shows an embodiment of a prioritization algorithm, where a datareduction method is applied and all critical data are sent beforenon-critical data. The data is separated in step 101 into critical data80 and non-critical data 82. While in step 84 the critical data has notall been sent, the wireless sensor unit 20 sends in step 86 criticaldata 80. Once all the critical data is sent, it sends in step 86non-critical data 82.

A three-dimensional seismic sensor produces data samples for the X, Y,and Z axis at a given sample rate per second. For immediate (short-term)analysis of the overall quality of the data record, only one dimension(say, the X axis) may be required. By assigning high priority to X axissamples, the data collection equipment is assured a much faster response(and analysis) of the data. Lower priority data (say the Y and Z axisdata) would be uploaded between shots or during pauses in active seismiccollection. FIG. 12 shows the original X, Y, and Z data 210, which havebeen separated into critical X data 220, and non-critical Y and Z data230.

The Usage is generally extensible to any triggered event that will causea significant increase in bandwidth usage due to the plurality ofdevices all attempting to communicate simultaneously (triggered by theevent), and where the data could be categorized according to short andlong term value.

Extensible Usage 1: Interlaced Data: Data to be uploaded is all of“equal value” but may be decimated for a lower resolution (but stilluseful) sample of the overall data quality. Decimation is performedbefore the wireless uplink and the remaining data are transmittedbetween shots or during pauses in active seismic collection. In FIG. 13,there is a 3-1 reduction in the amount of critical data sent, as theoriginal burst 310, is reduced by first selecting periodic or aperiodicdata 320, for initial transmission, then selecting the remaining data,330, for transmission at lower priority.

Extensible Usage 2: Reduction of Resolution: As shown in FIG. 14,initial upload data, 410, is reduced in size by trimming the lower orderbits 430 of each word, effectively reducing the resolution of theinformation. High order bits 420 are transmitted first, and theremaining bits are transmitted between shots or during pauses in activeseismic collection.

Extensible Usage 3: Combination of Interlacing, Reduction of Resolution(see FIG. 15): Data size is further reduced by a combination of thetechniques described above. The interlacing described above isperformed, and the reduction of resolution is used on the original data,510, to create high priority data 520, and low priority data 530. Thehigh priority interleaved data 540, is sent first, and then the lowpriority interleaved data 550 is sent later, as time permits.

Power Saving Methods

Conventional seismic data collection is performed through wiredconnections between the seismic sensors and the data collection andanalysis equipment. One issue with wireless replacement of datacollection cables is power consumption. Batteries power the remotesystems, but these batteries must be able to provide power for 2-3 daysof use, even in extremely low temperatures. Because the data processingdevices and radio equipment represent a large draw on the availablepower supply, it is critical to minimize power consumption whereverpossible. While there exist methods for reduction of power consumptionin radio devices such as cellular telephones, the characteristics of theseismic collection system present opportunities for unique new methodsfor power saving.

Power saving can be accomplished by turning off any unused devices atthe appropriate time. Prior art often focuses on methods of “wakening” asystem that is in low power consumption (or “sleep”) mode, includingtechniques where the device periodically wakes up and transmits amessage to see if the system should become active, or wakes up,receives, and decodes messages looking for indications that data isready to be transmitted or received. Embodiments of a wireless datanetwork are disclosed that allow power saving modes to operateefficiently.

Immediately following each seismic event (usually triggered by anexplosive device or other physical means to send a seismic shock waveinto the area being measured), a very large amount of data is ready fortransmission back to the data collection equipment. Because all of thesensors are triggered by the same event, this event can also be used toswitch the wireless portion of the system from sleep state to functionalstate. When a sensor detects an event in step 90, it triggers aprocessor interrupt in step 92 to take the radio out of sleep mode instep 94. The radio then transmits data in step 96 and then returns tosleep mode in step 98 (see FIG. 16). Because the radio system is notrequired to periodically check for network status, and because a givenradio may enter sleep mode immediately after it has finished uploadingits data, power save mode is much more efficient than conventionaltechniques.

Another technique stems from the fact that, at the end of the day, thesystem operator is aware that no more seismic tests will be conducted,and a single “sleep” message can be sent out through the network in step152, allowing all radios to shut down transmission and in step 154 setthe processors into a modified sleep mode. In this mode, the processormay awaken periodically in step 156 to sample the data at the radioreceiver (which consumes much less power than the transmitter). When theoperator is ready to use the wireless network, he transmits a signal tothe nearest node of the mesh. This node, upon receiving the “wake up”signal, wakes in step 158 and in turn begins to transmit the signal toneighboring nodes. In a ripple fashion, the network moves from a dormantstate to an active state 150 (see FIG. 17). Again, this is a novel wayto utilize the fact that the entire network should be switched todormant or active state at the same time.

In the claims, the word “comprising” is used in its inclusive sense anddoes not exclude other elements being present. The indefinite article“a” before a claim feature does not exclude more than one of the featurebeing present. Each one of the individual features described here may beused in one or more embodiments and is not, by virtue only of beingdescribed here, to be construed as essential to all embodiments asdefined by the claims. Immaterial modifications may be made to theembodiments described here without departing from what is covered by theclaims.

1. A wireless network, comprising: wireless sensor units organized ingroups of wireless sensor units, each wireless sensor unit comprisingplural sensors and at least a wireless transceiver connected tocommunicate by wires or wirelessly with the plural sensors; each groupof wireless sensor units including a terminal wireless sensor unit andintermediate wireless sensor units, each intermediate wireless sensorunit being configured to relay data through the intermediate wirelesssensor units towards the terminal wireless sensor unit; the terminalwireless sensor unit in each group of wireless sensor units beingadapted to communicate wirelessly with at least one backhaul unit ofplural backhaul units on a first set of frequencies; the backhaul unitsbeing adapted to communicate with a central computer on a second set offrequencies different from the first set of frequencies; and each groupof wireless sensor units operating on a different channel than adjacentgroups of wireless sensor units.
 2. The wireless network of claim 1 inwhich each group of the plural groups of wireless sensor units comprisesa linear array of wireless sensor units.
 3. The wireless network ofclaim 1 in which the wireless sensor units of at least one group arearrayed in a mesh pattern on a ground surface.
 4. The wireless networkof claim 1 in which wireless sensor units of each group of wirelesssensor units are divided into at least a first group and a second groupand the wireless sensor units of each group are configured so thatmembers of the first group transmit while members of the second groupreceive.
 5. The wireless network of claim 1 in which the wireless sensorunits comprise ground motion sensors.
 6. The method of claim 1 in whicheach backhaul unit of the plural backhaul units operates on a differentchannel from adjacent backhaul units.
 7. A method of collecting datafrom wireless sensor units arranged in a network, the wireless sensorunits being organized in groups of wireless sensor units, each group ofwireless sensor units including a terminal wireless sensor unit andintermediate wireless sensor units, the method comprising the steps of:initiating distribution of control signals to the wireless sensor units;acquiring data with the wireless sensor units by sensing one or morephysical parameters; each of the wireless sensor units transmitting theacquired data in response to the control signals through at least one ofthe plural groups of wireless sensor units towards a corresponding oneof the terminal wireless sensor units, each group of wireless sensorunits operating on a different channel from adjacent groups of wirelesssensor units; each of the terminal wireless sensor units forwarding theacquired data from the wireless sensor units in the corresponding groupto at least one of plural backhaul units, each backhaul unit of theplural backhaul units operating on a different channel from adjacentbackhaul units; and each of the backhaul units collecting and forwardingthe acquired data from the terminal wireless sensor units towards acentral computer, in which, for each group of wireless sensor units andbackhaul unit that the group of wireless units communicates with, thegroup of wireless sensor units communicates with the correspondingbackhaul unit on a first set of frequencies and the correspondingbackhaul unit communicates with the central computer on a second set offrequencies different from the first set of frequencies.
 8. The methodof claim 7 further comprising each wireless sensor unit delayingtransmitting of at least a portion of the acquired data according to aprioritizing algorithm associated with each respective wireless sensorunit.
 9. The method of claim 7 in which each wireless sensor unitcomprises plural sensors and at least a wireless transceiver connectedto communicate by wires or wirelessly with the plural sensors.
 10. Themethod of claim 7 in which the wireless sensor units in each group ofwireless sensor units are divided into at least a first group and asecond group and wireless sensor units of the first group transmit whilewireless sensor units of the second group receive.
 11. The method ofclaim 7 in which upon receiving a transmit control signal instructing atransmit of acquired data, wireless sensor units transmit acquired databefore distributing the transmit control signal to other wireless sensorunits.
 12. The method of claim 7 further comprising: placing wirelesssensor units in a sleep state during at least some time periods when thewireless sensor units are not acquiring data or transmitting acquireddata; and awaking the wireless sensor units from the sleep state with aninitiating control signal.
 13. The method of claim 12 in which thewireless sensor units comprise ground motion sensors and the initiatingcontrol signal is an initiation of a ground vibration.
 14. The method ofclaim 7 in which the wireless sensor units comprise ground motionsensors.
 15. The method of claim 7 in which each prioritizing algorithmcomprises a random delay in the transmitting of the acquired data. 16.The method of claim 7 in which each prioritizing algorithm comprisestransmitting a first portion of the acquired data before a secondportion of the acquired data.
 17. The method of claim 16 in which thefirst portion and the second portion have different resolution.
 18. Themethod of claim 17 in which the acquired data is transmitted as bytescomprising bits, and the first portion and the second portion correspondto different bits.
 19. The method of claim 7 in which each prioritizingalgorithm comprises transmitting different portions of the acquired databased on one or more of resolution and bit position.
 20. The method ofclaim 7 in which the groups of wireless sensor units are pre-set lineararrays.
 21. The method of claim 7 further comprising organizing thewireless sensor units by laying out the wireless sensor units in a meshand then self-organizing the wireless sensor units into the groups ofwireless sensor units.
 22. The method of claim 7 further comprisingorganizing the wireless sensor units by selecting for inclusion in eachgroup of wireless sensor units a number of wireless sensor units thatprovides transmission of data acquired in a data acquisition period tothe central computer within a time period equal to the data acquisitionperiod.
 23. The method of claim 7 in which one or more of the backhaulunits collect and forward the acquired data from the terminal wirelesssensor units towards a central computer through one or more relays.